In manufacturing semiconductor devices and integrated circuits, wafer-shaped or linear devices are commonly secured to a support, a substrate or a package. Various techniques have been devised to bond or cement such devices to a support, substrate or package.
U.S. Pat. No. 4,803,124 (R. Kunz), issued on Feb. 7, 1989, discloses a technique for bonding semiconductor chips to a mounting surface utilizing adhesive applied in starfish patterns. Kunz teaches depositing an adhesive material in a starfish pattern comprising a raised central portion disposed at the intersection of a plurality of radial arms. The bonding surface of the semiconductor chip is pressed against the adhesive deposit to cause the adhesive to uniformly and symmetrically spread therebeneath across the bonding surface and bond the two surfaces. Kunz also describes a prior art technique of placing five droplets of an adhesive in an "X" shape on a support surface using five hypodermic needles.
Typically in these techniques, a silver-filled thermally polymerizable conductive adhesive is disposed between the back surface of the device and the front surface of the package or substrate. Most of these techniques use a single continuous line of adhesive or a single row of spaced-apart dots of adhesive disposed along the long axis of the device for attaching the device to a support or substrate. The device is then placed on the adhesive by some means and properly aligned. The adhesive is then thermally cured to bond the device to the support or substrate.
Referring now to FIGS. 1 and 2, there is shown an exemplary arrangement of the above-described technique which uses a single row of dots of adhesive 12. More particularly, FIG. 1 is a front view showing a portion of a support, substrate or package 10, a dot of adhesive 12 (which is the first dot of adhesive 12 of the single row of dots 12 formed therebehind) disposed on the support or substrate 10, and a device 14 which has its long axis centrally positioned on the peaks of the row of dots of adhesive 12. FIG. 2 is a top view of the exemplary arrangement of FIG. 1 showing a non-continuous row of 16 dots of adhesive 12 (8 dots of adhesive 12 in each section), but it is to be understood that such row of dots of adhesive 12 could be continuous. It is to be further understood that for the alternative technique using a single continuous line of adhesive (not shown), the arrangement would look the same as the front view of FIG. 1, and only a side view and the top view of FIG. 2 would appear different in that the dots of adhesive 12 would be interconnected to form a continuous line of adhesive.
Referring now to FIG. 3, there is shown a result which can occur after the single row of dots of adhesive 12 of FIGS. 1 and 2 is thermally cured. As is illustrated in FIGS. 1 and 3, as the dots of adhesive 12 heat up during the curing process, the viscosity of the adhesive decreases and the adhesive flows to allow the device 14 to move and/or tilt uncontrollably depending on how the device 14 was positioned on the dots of adhesive 12. A similar result can occur using the single line of adhesive.
Such techniques are suitable for applications which do not require precise positioning of one or more devices 14 on the substrate or package 10. However, for some applications, each device 14 has to be precisely placed with a predetermined tolerance (e.g., at most .+-.15 microns) in the X and Y directions and not shift to exceed such tolerance on the thermal curing of the adhesive. As illustrated in FIGS. 1 and 3, the above-described conventional techniques are not useful for providing such precise positioning of the device 14 on the support, substrate or package 10.
Typically, when there is a need to assemble electronic devices (e.g., charge coupled integrated circuit chips) with high precision, the assembly is performed using elaborate and expensive manual techniques assisted with product-specific assembly fixtures. A need for product-specific fixtures typically produces inordinately high assembly costs. Additionally, any changes in product design are preceded by long lead times.
It has been a long sought after goal to provide a flexible and automated system of assembling (attaching) electronic devices to substrates using a bonding medium wherein the electronic devices are accurately located on the substrates. To be practical, such a system needs to be adaptable to the production of a wide range of product configurations. There are many problems associated with developing such an automated assembly system. For example, it is difficult to apply a bonding medium or adhesive to substrates in a manner that provides sufficient uniformity of support for the devices being assembled with the substrates.
Additionally, there is no known prior art way to automatically place a wide variety of devices (IC's) onto an adhesive coating on a substrate. Prior to curing the adhesive, the devices must be pressed onto the surface of the adhesive to assure that there is no movement of the devices during curing. The pressing of the devices onto the adhesive must be performed with a very carefully controlled force, typically in the range of between 5 to 20 grams with a tolerance of .+-.0.05 grams. A force that is too high causes the fragile devices to break. A force that is too low fails to properly seat the devices onto the adhesive. Because the devices have various thicknesses, it has been impractical to automatically control such a force within the limits required for successful assembly of the devices.
In prior art assembly methods, the devices were placed onto the adhesive coated substrates and the substrates were loaded into a high temperature oven to cure the bonding medium or adhesive. In spite of the exercise of extreme care in the formation of a uniform supporting base of adhesive and placement of the devices onto desired locations, the assemblies emerged from the oven with a slightly different positioning of the devices relative to the substrates. The cause of this variation of positioning was not fully understood, but it was assumed that the variations were caused by some physical distortions of the adhesive that occurred during curing.
After a careful and more insightful examination of this problem, it has been discovered that this undesirable phenomenon is a result of environmental vibration (i.e., vibrations caused by movements of vehicles and heavy objects in the vicinity of the curing operation or vibrations from sources such as fans in a curing oven) which produces lateral movement of the devices relative to the substrates while the adhesive is in a fluid state. There has heretofore been no recognition that such vibration is a problem.
It is desirable therefore to have an automated system for assembling devices to substrates using a bonding medium (e.g., adhesive) which results in the devices being accurately positioned and well secured to the substrate. Furthermore, the desired system should be adaptable to the production of a wide range of product configurations and produce high yields.